1931  Leica I (C) originally had non standardized lenses that were custom fitted because the distance from the lens flange to the film plane was not a standard distance. Later examples had the distance standardized and an 'O' was engraved on the lens mount to signify that the lens mount was standardized. This model is the first Leica with interchangeable lenses.  This standardized example has an early interchangeable Elmar with the infinity lock at the 11 o'clock position instead of the later 7 o'clock position.